Is Covid a disease of the blood vessels?


From reports of ‘sticky blood’ raising the risk of deep vein thrombosis, heart attack or stroke, to neurological effects, to painful red and swollen areas on the feet known as ‘Covid toe’ many of these symptoms are thought to be linked to effects on our blood vessels.
In particular, these symptoms may be caused by the way that Covid-19 affects the lining of the blood vessels, called the endothelium. This layer of cells is not a solid barrier can allow or block certain substances through depending on conditions in the body. And a healthy, well-functioning endothelium helps to keep our blood vessels relaxed and open to the flow of blood. It also releases substances that help to prevent harmful blood clots and inflammation. But if damaged, the processes may not work effectively.

In April 2020, a paper published in The Lancet gave the first evidence that SARS-CoV-2 can infect endothelial cells. The molecule ACE2 (which the virus binds to in order to enter our cells and reproduce itself) can be found on the surface of endothelial cells. Another study found higher ACE2 levels in lung endothelial cells, alongside evidence of severe injury to blood vessels. The formation of tiny clots within capillaries (our smallest blood vessels) in the lungs was nine times more in Covid-19 victims compared with the lungs of people who died from flu.

By damaging the endothelium, the coronavirus has the potential to cause abnormal blood clotting, ‘leaky’ vessels and reduced blood flow. In the lungs, these effects cause clots and fluid to accumulate, meaning the lungs are less able to get oxygen into the body.

In the brain, damage to endothelial cells in the blood-brain barrier could lead to inflammation. In limbs, reduced blood flow could lead to ‘Covid toe’. Further, overactivation of our inflammation systems can also make the blood more likely to clot.
This may explain why people with certain heart and circulatory conditions seem to be at a higher risk of developing severe complications. If another condition, such as diabetes or high blood pressure, has already damaged the endothelium, the virus’s impact is magnified.

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